Spinal pain has long been a source of patient discomfort and a limitation on the patient's mobility and quality of life. Spine fusion (arthrodesis) is a procedure in which two or more adjacent vertebral bodies are fused together. It is one of the most common approaches for alleviating various types of spinal pain, particularly pain associated with one or more affected intervertebral discs. While spine fusion generally helps to eliminate certain types of pain, it has been shown to decrease function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, the fusion creates increased stresses on adjacent non-fused vertebra, and accelerated degeneration of the vertebra. Additionally, pseudarthrosis (resulting from an incomplete or ineffective fusion) may not provide the expected pain relief for the patient. Also, the device(s) used for fusion, whether artificial or biological, may migrate out of the fusion site, creating significant new problems for the patient.
Various technologies and approaches have been developed to treat spinal pain without fusion in order to maintain or recreate the natural biomechanics of the spine. To this end, significant efforts are being made in the use of implantable artificial intervertebral discs. Artificial discs are intended to restore articulation between vertebral bodies so as to recreate the full range of motion normally allowed by the elastic properties of the natural disc. Unfortunately, the currently available artificial discs do not adequately address all of the mechanics of motion for the spinal column.
More recently, surgical-based technologies, referred to as “dynamic posterior stabilization,” have been developed to address spinal pain resulting from one or more disorders, particularly when more than one structure of the spine has been compromised. An objective of such technologies is to provide the support of fusion-based implants while maximizing the natural biomechanics of the spine. Dynamic posterior stabilization systems typically fall into one of two general categories: posterior pedicle screw-based systems and interspinous spacers.
Examples of pedicle screw-based systems are disclosed in U.S. Pat. Nos. 5,015,247; 5,484,437; 5,489,308; 5,609,636; 5,658,337; 5,741,253; 6,080,155; 6,096,038; 6,264,656; and 6,270,498. These types of systems typically involve the use of screws that are positioned in the vertebral body through the pedicle. Because these types of systems require the use of pedicle screws, implanting the systems is often more invasive than implanting interspinous spacers.
Examples of interspinous spacers are disclosed in U.S. Pat. Nos. Re 36,211; 5,645,599; 6,149,642; 6,500,178; 6,695,842; 6,716,245; and 6,761,720. The spacers, which are made of either a hard or a compliant material, are placed between the adjacent spinous processes of adjacent vertebra. While slightly less invasive than the procedures required for implanting a pedicle screw-based dynamic stabilization system, hard or solid interspinous spacers still require that the muscle tissue and the supraspinous and interspinous ligaments be dissected. Accordingly, in some instances, compliant interspinous spacers are preferred. However, the compliancy of such spacers makes them more susceptible to displacement or migration over time. One type of spacer developed by the assignee of the present application, and disclosed in U.S. patent application Ser. No. 11/314,712, is directed to rigid interspinous spacers that may be deployed from a posterior direction so as to reduce the amount of tissue dissected during implantation. These spacers also include deployable features that are stowed as the spacer is implanted to provide a low profile shape, and are then expanded once the spacer is implanted to provide the structure that stabilizes neighboring vertebra. Such devices have proven beneficial in many instances. However, there remains a need for reducing the invasiveness of an interspinous implant, while at the same time (a) reducing the likelihood for the implant to migrate, and (b) maintaining or improving the ability of the implant to provide suitable stability.